Implemented a simple SGI

In order to handle FastCGI, I had to split the libsoup code from Request and Response objects.

This has a fully working `libsoup` implementation and prepares the land for `FastCGI` integration.

Major changes:

* Request and Response body are now `DataStream`, which allow template streaming
* SGI.Application which decouple server handling from `Router`

These changes preserve most of the old interface, except for bodies handling as they are now streams.

I need some code review on the libsoup implementation as I had to write an MultiSet adapter over the MessageHeaders. These changes might need some unit tests as they are error-prone.